Some known apparatus for detecting light make use of the sensitivity of a semiconductor to light. Other known apparatus for detecting light use a photosensitive material. Still other known apparatus for detecting light utilize a photoelectric effect.
Some light-detecting apparatus which have been widely known employ a photovoltaic effect which converts a light energy into an electrical energy. Other light-detecting apparatus which have been widely known make use of photoconductivity, i.e., the conductivity of a semiconductor varies when illuminated with light. It is known that the former apparatus include photoelectric devices using a PN junction or PIN junction of silicon semiconductor. It is also known that the latter apparatus include photodiodes and phototransistors.
A known method for detecting ultraviolet radiation uses an alkalide phosphor that responds to ultraviolet radiation. Also, photomultipliers are known. Although these respond to UV light, they are unable to record information contained in UV light.
Also, a UV detector utilizing a diamond has been proposed, and its photosensitivity has been measured. However, this apparatus makes use of the photosensitivity of the diamond to UV light, i.e., variations in the electrical conductivity of the diamond induced by UV illumination. Again, this apparatus is unable to store information contained in UV light impinging on this apparatus.
Techniques for measuring the dose of illuminating light or radiation by exploiting light emission from the illuminated material are known and called optically stimulated luminescence (OSL) (Solid State Physics, 2, No. 324, Vol. 28, 1993, pp. 49-58).
This OSL is caused in the manner described below. BaFBr doped with Eu or RbBr doped with Tl is illuminated with UV light or visible light. The BaFBr or RbBr is further illuminated with infrared radiation that acts as stimulating light. The amount or intensity of light emitted corresponds to the amount of the UV or visible light emitted first.
The above-described optically stimulated luminescence can be roughly understood from a model described below. BaFBr or the like is doped with Eu that is a phosphor. UV light having energies greater than the bandgap energy of the phosphor is made to hit the BaFBr, so that electron-hole pairs are generated inside the phosphor. Some of the electron-hole pairs are trapped in lattice defects and in the centers of capture of positive holes or electrons which are created by impurities. Since these centers of capture are located deep in the bandgap, electrons and positive holes captured in the centers are in stable state even at room temperature. The levels of the energies at which the centers of capture exist correspond to the energies of the light in the infrared region. Accordingly, light at these levels is directed as stimulating light to the phosphor. In this way, electrons and positive holes captured in the centers of capture recombine, thus emitting light.
The intensity of the emitted light is in proportion to the amount of the illuminating UV light. Therefore, in principle, it should be possible to read and write the amount of the UV light by making use of this phenomenon. However, when one attempts to measure the amount of the illuminating UV light by making use of this phenomenon, the intensity of light emitted from the phosphor is so weak that the method is not practical. The intensity of the emitted light may be effectively increased by increasing the thickness of the phosphor film. If the phosphor film thickness is increased, the intensity of the illuminating UV light and the intensity of the stimulating light for reading the amount of illuminating UV light must be increased. If the intensity of the reading light is increased, local heating occurs. The produced heat diffuses to the surroundings. This deteriorates the resolution at which information is read out. If the thickness of the film of the phosphor is increased, scattering of the light is increased. This also leads to a decrease in the resolution at which information is read and written.
In this way, the detection of UV light utilizing optically stimulated luminescence (i.e., the stimulating light is emitted, light is generated, and yet the generated light must be observed) suffers from fundamental difficulties. In consequence, this method has low practicability.
The background of the present invention is further described below. Referring to FIGS. 2, (a) and (b), there is shown a UV detector. FIG. 2(a) is a top view of the UV detector. FIG. 2(b) is a cross-sectional view taken on line A-A' of FIG. 2(a). This detector comprises a thin diamond film 21, a pair of electrodes 22, 23, and a pair of output electrodes 24, 25.
FIG. 1 shows the relation of logarithmic values of the photosensitivity of the diamond film 21 of the UV detector shown in FIGS. 2, (a) and (b), to the photon energy of light impinging on the diamond film 21. Also, the relation of the photon energy of the light impinging on the diamond film to the transmission, or transmittance, of the diamond film is shown. In the graph, the photon energy h.nu. is plotted on the horizontal axis. In practice, however, the wavelength of the light corresponds to the transmission. That is, the wavelength of the light is converted into an energy expressed in electron volts (eV) and plotted on the horizontal axis.
In FIG. 1, white circles indicate the dependence of the transmission of the diamond film on wavelength, which should be read on the right scale. Black triangles are obtained by illuminating the diamond film with light of wavelengths of 180 to 380 nm corresponding to energies of about 6.9 to 3.3 eV, and then measuring the photosensitivity of the diamond film 21. The light of wavelengths of 180 to 380 nm whose corresponding energies are plotted on the horizontal axis is produced by a deuterium discharge lamp, the intensity of the light being 15 .mu.W/cm.sup.2. Black circles indicate the photosensitivity when only light having wavelengths plotted on the horizontal axis is emitted without illuminating the deuterium discharge lamp.
In the present patent specification, x E(.lambda.)=1240 is used as an approximation formula for converting wavelength (nm) into photon energy E(.lambda.) (in eV).
We now take notice of the white circles. It can be seen that the transmission drops rapidly from around 5.5 eV. This indicates that the energy bandgap of the thin diamond film is approximately 5.5 eV corresponding to a wavelength of about 230 nm. With respect to black circles, the photosensitivity increases with increasing the photon energy of the illuminating light, i.e., as the wavelength of the illuminating light shortens.
With respect to the black triangles, the photosensitivity is almost constant at wavelengths having energies exceeding about 2.8 eV corresponding to a wavelength of about 440 nm. Above about 5 eV corresponding to a wavelength of about 250 nm, the black triangles assume values close to the values indicated by the black circles which are obtained when the deuterium discharge lamp is not lit up. Comparing the results (indicated by the black triangles) of measurements of the photosensitivity made while the deuterium discharge lamp is producing UV light with the results (indicated by the black circles) of measurements of the photosensitivity made without the UV irradiation shows that their trajectories are almost the same above about 5 eV and differ widely below about 5 eV. The energy of about 5 eV is quite close to the energy bandgap of diamond, or about 5.5 eV. We observe that both values are almost coincident.
It can be seen from FIG. 1 that the photosensitivity to light of energies less than about 5 eV corresponding to wavelengths of about 250 nm and above is affected by the illumination of the UV light from the deuterium discharge lamp. Thus, we can understand that the photosensitivity obtained when light of energies less than about 5 eV contains information about illumination of wavelengths exceeding 5 eV of the UV light emitted from the deuterium discharge lamp.
We can conclude from the foregoing that when the diamond is illuminated with UV light having wavelengths shorter than the wavelength approximately corresponding to the energy bandgap of the diamond, the information can be taken from the photocurrent by illuminating the diamond with light having wavelengths longer than the wavelength substantially corresponding to the energy bandgap of the diamond.
FIG. 3 shows the relation of the photocurrent to the UV irradiation time when the device shown in FIGS. 2, (a) and (b), is illuminated with light having wavelengths of 180 to 350 nm, the light being emitted from the deuterium discharge lamp. That is, FIG. 3 indicates the dependence of the photocurrent induced in the UV detector shown in FIGS. 2, (a) and (b), on time for the UV light having wavelengths of 180 to 350 nm. At this time, the photocurrent induced in the diamond film 21 is measured by applying a certain bias voltage of about 10 V between the electrodes 22 and 23 which are spaced 0.7 mm from each other while illuminating the thin diamond film 21 with the light from the deuterium discharge lamp. The photocurrent which is of the order of picoamperes is amplified by an operational amplifier and converted into a voltage by a resistor of 1 G.OMEGA.. This voltage is produced from a plotter. Actual values are obtained from this plotter output.
Three curves are shown in FIG. 3. The curve indicated by the white circles was obtained when UV light of intensity 34 .mu.W/cm.sup.2 was emitted. The curve indicated by the black triangles was obtained when UV light of intensity 15 .mu.W/cm.sup.2 was emitted. The curve indicated by the black dots (interposed by curves) was obtained when UV light of intensity 7 .mu.W/cm.sup.2 was emitted.
The right end of the curve indicated by the black dots indicates that the pulsed photocurrent flowed. This curve was derived by illuminating the thin diamond film 21 with white light having wavelengths of 360 to 800 nm after the end of illumination of UV light from the deuterium discharge lamp and measuring the induced photocurrent.
It can be seen from FIG. 3 that the saturated value of the photocurrent varies, depending on the intensity of the illuminating light from the deuterium discharge lamp. Data about this relation is shown in FIG. 4, where logarithmic values of the intensity of illuminating light from the deuterium discharge lamp is plotted on the horizontal axis, while the logarithmic ratio of the photocurrent induced when the UV light is not emitted to the photocurrent saturated by illumination of the UV light is plotted on the vertical axis. That is, FIG. 4 shows the relation between the UV radiant intensity when the UV detector shown in FIGS. 2, (a) and (b), is illuminated with UV light and the ratio of the photocurrent in illuminated state to the photocurrent in dark state. It can be seen from FIG. 4 that a clear proportional relationship exists between the intensity of UV light and the saturated photocurrent over a range of about three orders of magnitude.
In FIG. 5, the intensity of the UV light emitted from the deuterium discharge lamp is plotted on the horizontal axis, while the time taken for the photocurrent to reach 90% of the saturated level is plotted on the vertical axis. It can be seen from FIG. 5 that when the intensity of the UV light is weak, it takes a long time for the photocurrent to become saturated. However, when the intensity of the UV light is strong, it takes a short time for the photocurrent to become saturated.
The right side of the graph of FIG. 3 shows variations in the photocurrent occurring when the thin diamond film 21 is illuminated with white light after the film 21 is sufficiently illuminated with the UV light from the deuterium discharge lamp at a radiant intensity of 7 .mu.m/cm.sup.2. Line (a) of FIG. 6 shows the relation between the peak value (in arbitrary units) of the photocurrent induced by illumination of white light after emission of UV light and the amount of UV light (in .mu.Ws/cm.sup.2) emitted from the deuterium discharge lamp. Line (a) is plotted on the left scale. Line (b) of FIG. 6 shows the relation between the total area (in arbitrary units) under the photocurrent curve when white light is emitted for 5 minutes and the amount of UV light from the deuterium discharge lamp. Line (b) is plotted on the right side.
The amount of light is defined as the radiant intensity x the irradiation time and also known as dose. The amount of light is expressed in .mu.Ws/cm.sup.2. The total amount under the photocurrent curve is obtained by integrating the photocurrent induced by white light illumination shown in FIG. 3 for 5 minutes with respect to time. In FIG. 6, relative values of the total amount are shown.
It can be seen from line (a) of FIG. 6 that a proportional relation exists between the UV dose of the thin diamond film 21 and the peak value of the photocurrent induced when the film 21 is illuminated with white light after UV illumination. It can be seen from line (a) of FIG. 6 that points (indicated by the black circles, the black triangles, and the white circles) corresponding to values of the radiant intensity of UV light are roughly located on a straight line. In consequence, the peak value of the photocurrent induced by the illumination of the white light precisely reflects the dose rather than the radiant intensity of the UV light.
It can be seen from line (b) that a proportional relation exists between the amount of UV light, or dose, impinging on the thin diamond film and the total amount of the photocurrent induced when the diamond is illuminated with white light for 5 minutes after UV illumination. It can be understood that the UV dose can be found from the total amount of the photocurrent induced by the illumination of the white light.
We can conclude from the experimental data shown in FIG. 6 that the amount of the UV light which is emitted from the deuterium discharge lamp and falls on the thin diamond film 21 is determined either from the peak value of the photocurrent induced in the diamond film 21 by illumination of the white light or from the total amount of the photocurrent obtained by integrating the photocurrent for a given time. That is, the white light can be used as reading light for reading the amount of UV light impinging on the diamond film.
The peak value of the photocurrent induced by illumination of white light can be considered to be the total amount of the photocurrent within an infinitesimal time which is obtained by integrating the photocurrent for an infinitesimal time. If the infinitesimal time is defined as a given time, measurement of the peak value of the photocurrent is essentially the same as measurement of the total amount of the photocurrent within a given time.
As indicated on the right side of FIG. 3, when white light is emitted after UV irradiation and the peak value of the induced photocurrent is measured, the peak value decreases slowly as the white light is emitted. Thus, we observe that information about the emitted UV light is read out and erased as the diamond is irradiated with white light.
In the example shown in FIG. 3, white light having wavelengths ranging from 360 to 800 nm is used as reading light. Data obtained when light having a single wavelength is used as the above-described reading light is shown in FIG. 7. In FIG. 7, energies (in eV) converted from wavelengths are plotted on the horizontal axis, whereas the time taken for the photocurrent induced when the reading light is emitted to decrease to 10% of its peak value is plotted on the vertical axis. To obtain the single wavelength, a spectroscope is used.
It can be seen from FIG. 7 that as the energy of the wavelength of the reading light for reading the amount of the UV light decreases, i.e., as the wavelength is increased, the photocurrent decreases more slowly. As the energy of the wavelength of the reading light is increased, the photocurrent decreases more rapidly. We can understand that as the energy of the reading light for reading the amount of UV light decreases, i.e., as longer wavelength is used, the reading time is prolonged. Conversely, if the wavelength has a larger energy, the reading time is made shorter.
When the reading light is omitted continuously, the photocurrent decreases gradually as shown on the right side of FIG. 3 though the rate of decrease is affected by the wavelength. After the reading light is emitted sufficiently, i.e., after the photocurrent has decreased sufficiently, if illumination of the reading light is once made to cease and then the illumination is restarted, then a photocurrent as shown in FIG. 3 no longer flows. This means that illumination of a sufficient amount of reading light destroys information regarding the amount of illuminating UV light. In the example shown in FIG. 3, white light is used as the reading light. In this case, the information is read out in a short time, as shown in FIG. 3. However, utilizing this phenomenon, information written by UV light can be erased.
Where pulsed light of a short duration is used as reading light, the peak value of the photocurrent induced by the illumination of the reading light has a proportional relation with the UV dose, as shown also in FIG. 6. In this case, whenever each pulse is irradiated as reading light, the peak value of the photocurrent correctly reflecting the UV dose can be obtained. Hence, the UV dose can be read out plural times. Of course, if the reading is repeated, the peak value of the photocurrent induced by irradiation of the reading light gradually decreases although at varying rates.
The number of these reading operations can be roughly known from the data shown in FIG. 7. As an example, light having a wavelength of about 520 nm corresponding to energy 2.4 eV is used as reading light. It is assumed that the irradiation time required for one reading operation is on the order of milliseconds. The amount of the UV light impinging on the diamond film can be known by performing more than 10.sup.3 reading operations. If it is only necessary to know whether UV light is irradiated, more than 10.sup.4 reading operations can be carried out. Of course, if the wavelength of the pulsed light for reading is shortened, then the number of reading operations for precisely reading the amount of the UV light decreases with the tendency shown in FIG. 7, where the number of reading operations is plotted on the vertical axis, and the values of energies corresponding to wavelengths of pulsed light are plotted on the horizontal axis.
Where pulsed light is used as reading light, if the radiant intensity is large, the number of accurate measurements of the UV dose is small. Conversely, if the radiant intensity is small, the number of measurements is large.
The fundamental data shown in FIGS. 1 and 3 to 7 have been obtained where a deuterium discharge lamp having a wavelength range from 180 nm to 350 nm is used as a UV light source. Therefore, we consider that when information regarding the amount of the UV light is written into the thin diamond film, the fundamental data is simultaneously read out with the reading light having wavelengths longer than about 230 nm corresponding to the energy bandgap of the diamond. Since the radiant intensity spectrum of the deuterium discharge lamp used in this experiment has takes such a form that it is stronger at shorter wavelengths and weaker at longer wavelengths. Therefore, we think that the effects of light having wavelengths longer than about 230 nm corresponding to the energy bandgap of the diamond are small.
To confirm this consideration, the deuterium discharge lamp was illuminated. At the same time, a mercury lamp having large energies at wavelengths longer than 230 nm was illuminated. Then, white light was emitted. The resulting photocurrent was measured. We have confirmed that this photocurrent is weak. In this case, therefore, we understand that the light having wavelengths longer than about 230 nm greatly affects the emission of light having wavelengths shorter than about 230 nm. On the other hand, the effects of light from the deuterium discharge lamp, i.e., having wavelengths longer than about 230 nm, are small. A different experiment was conducted. In particular, only the mercury lamp was lit up without illuminating the deuterium discharge lamp. Then, white light was emitted, and the resulting photocurrent was measured. This measured photocurrent was similar to the photocurrent shown in FIG. 3 but had smaller values. This did not enable measurement of the amount of light from the mercury lamp. It is considered that almost all wavelengths contained in the light emitted from the mercury lamp have energies smaller than about 5.5 eV that is the energy bandgap of diamond. This phenomenon also occurs where white light is emitted simultaneously with illumination of the mercury lamp.
It has been confirmed that in an ordinary room or in a room illuminated with light (which can be regarded as white light) of intensity comparable to outside brightness, data shown in FIGS. 3 and 6 are obtained with high reproducibility. That is, even where reading light is emitted, if the center of the spectrum of the reading light has a wavelength corresponding to the energy much lower than the bandgap of the material, then illumination of writing light does not impede integration of the amount of illuminating light. Therefore, where diamond is used, detection of UV light in an ordinary room or outside can be selectively done without the need of a filter or the like.
What are obtained from the above considerations are summarized below.
(1) Information regarding UV light having photon energies exceeding the energy bandgap of diamond can be measured by measuring the peak value of the photocurrent induced by reading light having photon energies less than the energy bandgap of the diamond. That is, information about the UV light having wavelengths shorter than about 230 nm impinging on the diamond can be read by illuminating the diamond with the reading light having wavelengths longer than about 230 nm.
(2) A plurality of reading operations can be performed by using pulsed light as the reading light.
(3) The number of reading operations can be increased by using a longer wavelength of light as the reading light.
(4) The number of the reading operations can be increased by using reading light of lower intensity.
(5) Information regarding the amount of the illuminating UV light can be erased by using white light as reading light, increasing the amount of the reading light, or reading out the stored information slowly.
In the item (1) above, the information about UV light means information about at least one of the amount of the UV light impinging on the diamond material, the presence or absence of irradiation, and difference in wavelength.
In order to understand the mechanism by which information about the amount of the illuminating UV light is stored in the diamond, a model of operation is now considered. Referring again to FIG. 1, the diamond is illuminated with light having energies less than about 5 eV to induce a photocurrent. We can understand that UV light impinging on the diamond and having energies exceeding about 5 eV produced effects. Also, we can see that points indicated by the black circles and points indicated by the triangles have different photosensitivities at less than about 5 eV and that the difference appears over the whole energy less than about 5.5 eV. Thus, we can consider that information concerning the illuminating UV light is distributed over the whole bandgap of about 5.5 eV of the diamond. Accordingly, we consider the following model and experimental data shown in FIGS. 1 and 3 to 7.
(a) Numerous impurities and lattice defects exist in a diamond crystal, and these form trap levels over the whole energy bandgap.
(b) Accordingly, UV light for exciting electrons in the diamond crystal is made to hit the crystal to optically induce electrons and positive holes. The electrons and the positive holes are captured in the trap levels in such a manner that they correspond to information about the amount of the illuminating UV light.
(c) If light having an energy necessary to excite them out of the trap levels, i.e., light having an energy less than the energy bandgap of the diamond, is emitted as reading light, carriers captured in the trap levels are excited. If a bias voltage is applied, the excited carriers produce a photocurrent. The amount of the UV light can be determined from this photocurrent. It can be seen from the data shown in FIG. 6 that the total amount of the photocurrent in a given time reflects the number of carriers excited out of the trap levels by UV illumination. That information about the amount of UV light is read out and erased with white light as shown in FIG. 3 means that the carriers captured in the trap levels have been all excited into carriers contributing to the photocurrent by UV illumination.
In the above-described experiment using a mercury lamp, the amount of light from the mercury lamp cannot be read from the photocurrent induced by illumination of white light. The reason why the amount of light cannot be read from the photocurrent may be understood as follows.
The spectrum of the radiant intensity of a mercury lamp is largely in a wavelength range exceeding about 230 nm, and the intensity of the spectrum is strong between about 250 and 400 nm. The spectrum contains very weak wavelengths corresponding to energies higher than the bandgap of diamond. Carriers are trapped in trap levels by these weak wavelengths and simultaneously excited by the wavelengths longer than about 230 nm. In other words, the apparent number of carriers trapped in the trap levels is reduced greatly.
In any case, the photocurrent which is attributed to the carriers trapped in the trap levels and induced by white light is quite weak. As a result, the value of the photocurrent fails to correctly reflect the amount of light emitted from the mercury lamp.
Where the diamond is illuminated with white light and also with the light from the mercury lamp and then the resulting photocurrent is measured, the amount of light from the lamp can also not be determined from the photocurrent. One would be able to understand that the reason why the photocurrent cannot be determined from the amount of light is that the number of carriers trapped in the trap levels is quite few.
The results of our consideration of the experimental data shown in FIGS. 3 to 7, using the model described above, are described below.
(a) As UV light hits the diamond, the trap levels are gradually buried by electrons and finally saturated.
(b) We consider that the value of the photocurrent induced by illumination of reading light is associated with at least one of the number of trap levels in which carriers are captured and the depths of the trap levels.
(c) We think that the rate at which the trap levels are buried depends on the total energy of UV light.
(d) We consider that the number of carriers trapped in the trap levels is determined by the total energy of the UV light impinging on the diamond.
(e) We consider that the photocurrent induced by illumination of white light depends on at least one of the number of trap levels in which carriers are captured and the depths of the trap levels. That is, the photocurrent induced by the white light illumination indicates the total energy of the UV energy impinging on the diamond.
(f) UV light is considered to have two kinds of energies: its radiant intensity and energies possessed by its wavelengths. If this is taken into consideration, the number of electrons trapped in the trap levels should differ, of course, according to the energy of each wavelength where the amount of UV light is set to a given value and the wavelength of the UV light is varied.
(g) In this case, therefore, the value of the photocurrent induced by the illumination of reading light should vary, depending on the wavelength of the UV light.
We conclude from the above considerations that variations in wavelength of UV light first directed to a diamond can be detected by setting the amount of the UV light to a given value, irradiating the diamond with UV light rays having different wavelength regions separately, and measuring the photocurrent induced by illumination of reading light. We understand that in the experiments giving rise to the data shown in FIGS. 1 and 3 to 7, UV light having the same radiant intensity spectrum, or the same wavelength region, emitted from a deuterium discharge lamp is used, and the total UV dosage of the diamond is varied by varying the amount of the UV light.
We have obtained experimental data which supports the results of the above considerations. In particular, light emanating from a deuterium discharge lamp was divided into certain wavelength regions by a spectrograph. Using the spectrally divided UV light rays, the relation between the wavelength of the UV light and the value of the photocurrent induced by the irradiation of reading light, or white light, was examined. We have confirmed that UV light having a wavelength of 180 nm and UV light having a wavelength of 200 nm produced different values of photocurrent in response to the irradiation of the reading light. Of course, the two kinds of UV light were made coincident in light amount. However, when the light from the deuterium discharge lamp was spectrally split, the radiant intensity was reduced greatly, i.e., by a factor of more than 10. Hence, a sufficiently large photocurrent could not be produced in response to the reading light. As a result, we could not obtain a clear relation between the wavelength of UV light and the photocurrent induced by illumination of the reading light.
We further discuss the model described above. We conclude that materials to which the model can be applied must satisfy the following requirements:
(a) They show photoconductivity. That is, they have sensitivity to light.
(b) They have some form of trap levels which trap excited electrons.
(c) They have energy bandgaps.
All of materials which are called semiconductors intrinsically possess the above-described physical properties.
Accordingly, we have been led to the recognition that materials which are different from diamond but have the physical properties (a)-(c) described above respond to light as shown in FIGS. 1 and 3 to 7 in principle, and that their nature can be exploited. In other words, the present invention has been made, based on the recognition that materials which are different from diamond but have the physical properties (a)-(c) described above produce experimental data shown in FIGS. 1 and 3 to 7 in principle, and that their nature can be exploited.
Where materials having the physical properties (a)-(c) are used, the contents of the items (1)-(5) described above are modified, based on the recognition described above, as follows.
(1') Information about light impinging on a material and having photon energies larger than the energy bandgap of the material can be known from the photocurrent produced in response to reading light having photon energies smaller than the energy bandgap of the material.
(2') A plurality of reading operations can be performed by using pulsed light as the reading light.
(3') The number of reading operations can be increased by using a longer wavelength of light as the reading light.
(4') The number of the reading operations can be increased by using reading light of lower intensity.
(5') Information stored in the material can be erased by reading out the information. For this purpose, the material is illuminated with reading light having an energy lower than the energy bandgap of the material in such a way that (i) the light has a considerable range of wavelengths, i.e., the total energy is large, (ii) the light amount is considerably large, i.e., the total energy is large, or (iii) the illumination time is long, i.e., the total energy is large.
The aforementioned information about light described in the item (1') above is information concerning at least one of the amount of the light impinging on the material, the presence or absence of irradiation, difference in wavelength, and difference in radiant intensity spectrum of the light having a wavelength corresponding to a photon energy greater than the energy bandgap of the material. Of course, if the total energy differs, the number of trapped carriers varies.
It is an object of the present invention to provide a device capable of detecting various amounts of light and various wavelengths of light.
It is another object of the invention to provide a device capable of storing various amounts of light and various wavelengths of light.
It is a further object of the invention to provide an electronic apparatus capable of performing arithmetic operations using light.
It is a yet other object of the invention to provide a device from which information stored therein can be readily read.
These objects are achieved in accordance with the teachings of the invention by a method consisting of illuminating a material satisfying certain conditions with first light to write information corresponding to the total energy of the first light. When the information is stored in the material, the information is read from the material by illuminating it with second light which satisfies given conditions by reading the information from a pulsed photocurrent induced in the material.
The aforementioned material satisfying the certain conditions has photoconductivity, an energy bandgap, and trap levels. An example of this material is a thin polycrystalline diamond film fabricated by a vapor phase method.
The aforementioned first light which satisfies the given conditions is required to have photon energies greater than the energy bandgap of the material satisfying the certain conditions. The aforementioned second light satisfying given conditions is required to have photon energies smaller than the energy bandgap of the material satisfying the certain conditions.
An example of the total energy of the first light is the amount of light (radiant intensity.times.irradiation time) which corresponds to the total irradiant energy per unit area expressed, for example in .mu.Ws/cm.sup.2.
A first aspect of the present invention lies in an electronic apparatus having a material which is illuminated with first light and illuminated with second light to induce a pulsed photocurrent in the material, thereby reading information about the first light. The material has photoconductivity, an energy bandgap, and trap levels. The first light has photon energies greater than the energy bandgap of the material. The second light has photon energies smaller than the energy bandgap of the material.
This apparatus makes use of the facts that photoconductivity is induced in the material by illumination of the second light and that its electrical resistance changes. At this time, a bias voltage is applied to the material. In this way, a pulsed photocurrent can be obtained. Information written with the first light can be read out by measuring the photocurrent. We can consider that measurement of the photocurrent is a means for measuring the resistance value attributed to the photoconductivity. In the configuration described above, the first light acts as writing light for writing information into the material. The second light serves as reading light for reading information from the material. The second light also acts to take out, or erase, information from the material.
As described already in the Background of the Invention, the present invention is based on the result of our experiment, i.e., the amount of the first light can be known from the photocurrent induced in the material by irradiation of the second light. When the apparatus is operated in practice, the maximum value, or peak value, of the pulsed photocurrent shown in FIG. 3 is measured, or the photocurrent is integrated with respect to time for a given time. The value obtained by integrating the photocurrent is given by .intg.I(t)dt, where I(t) is the photocurrent. This value corresponds to the number of carriers promoted out of the trap levels in the material.
The photoconductivity means the following phenomenon. Electron-pair holes are generated by illumination of light. These electrons or positive holes act as carriers for conduction of electricity. The electrical resistance drops, i.e., the conductivity increases.
Examples of the above-described material which has photoconductivity, an energy bandgap, and trap levels include thin films of polycrystalline diamond. Other preferable materials are single-crystal diamond, SiC, and other semiconductor materials having wide bandgaps such as boron nitride, ZnO, ZnS, ZnSe, ZnTe, CdS, and AlN. Of course, other compound semiconductors having wide bandgaps may also be used. The conductivity types of these semiconductors can be any one of P type, I type, and N type. An appropriate type can be selected according to the need. Furthermore, impurities can be added to control their electrical characteristics such as conductivity and bandgap.
Also, Si, Ge, Se, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, InSb, PbO, CsSe, CdTe, other compound semiconductors, and these materials to which impurities have been added can be employed. However, materials which are actually used preferably have a wide bandgap and a strong crystalline structure such as diamond. As an example, amorphous silicon is not desired because its physical properties are deteriorated when illuminated with intense light. Specifically, when the material is illuminated with second light, or reading light, the resistance of the material is varied by carriers excited out of the trap levels. In addition, the resistance is varied by a change in the quality of the material, e.g., deterioration or a change in the crystal structure such as crystallization, in other words, a structural change at a molecular level. As a result, the effects cannot be neglected.
A second aspect of the present invention is based on the first aspect described above and characterized in that the material is irradiated with second light after irradiated with the first light.
In the first aspect, the relation between the timing at which the first light is emitted and the timing at which the second light is emitted is not clear. When cause and effect are taken into account, it is obvious that the first light is not emitted after illumination of the second light. In this second aspect, the second light is illuminated after illumination of the first light so that information about the illumination of the first light is obtained.
A third aspect of the invention is based on the first aspect described above and characterized in that the material is illuminated with the first and second light simultaneously. In this scheme, information about the first light emitted before the illumination of the second light can be obtained.
A fourth aspect of the invention is based on the first aspect described above and characterized in that information about the first light impinging on the material is the amount of the first light impinging on the material. This amount of the first light falling on the material is read by illuminating the material with the second light, as indicated by the data obtained from a polycrystalline diamond, the data being shown in FIG. 6.
For example, a pair of electrodes are installed on the material. A bias voltage is applied between the electrodes. Under this condition, the first light is projected between the electrodes. Then, the material is illuminated with the second light. A photocurrent is induced between the electrodes. In this way, the relation between the amount of the first light and the peak value of the photocurrent can be known. Alternatively, the relation between the amount of the first light and a value obtained by integrating the photocurrent with respect to time can be known.
The photocurrent induced by the illumination of the second light takes a pulsed form as shown in FIG. 3. When the current is measured in practice, its peak value is measured, or the current is integrated for a given time. Then, the amount of the first light is calculated, using the relation (FIG. 6) previously set.
We can understand that the pulsed photocurrent measured as described above is a value obtained by integrating the photocurrent for an infinitesimal time ?t immediately after the photocurrent begins to flow. Also, we can consider that measurement of the photocurrent (I) is a measurement of the amount of charge, i.e., .intg.I(t)dt=Q. We can understand that this is a measurement of the number of carriers excited out of the trap levels in the material by the second light. The apparatus operating as described above can be regarded as a memory which receives the amount of the first light. Of course, the simplest operation is to know whether the material is illuminated with the first light or not by measuring the photocurrent induced by the second light. That is, binary data 0 and 1 can be treated.
The amount of light impinging on the material is defined as the product of the radiant intensity and the irradiation time. If the radiant intensity I depends on time, the amount of light is given by .intg.I(t)dt. For example, the amount of light is expressed in .mu.Ws/cm.sup.2. In the present specification the concept of the amount of light impinging on the material is chiefly used. If the irradiation time is limited to a unit time, the same discussion can be made, using the concept of the radiant intensity.
If the material is irradiated with the second light while irradiated with the first light, the integrated amount of the first light which is obtained by integrating the radiant intensity for an irradiation time before the illumination of the second light can be known from the value of the photocurrent.
A fifth aspect of the invention is based on the first aspect and characterized in that information about the first light impinging on the material relates to different wavelengths of the first light.
As already described in the Background of the Invention, we can conclude that what can be read with the second light is not limited to the amount of the first light. Information regarding different wavelengths can also be read. For example, where two kinds of light having the same amount of light but different wavelengths are used, different photocurrents are induced by them because their photon energies (h.nu.) are different from each other. It is possible to judge which of the two kinds of light was emitted.
A sixth aspect of the invention is based on the first aspect described above and characterized in that the information about the first light impinging on the material is represented by combinations of wavelengths and amounts of the first light. For example, where the first light consists of two wavelengths of light having different irradiant intensities, plural combinations of these wavelengths and irradiant intensities can be used as information.
Preferably, the first light used in the above aspects has a single wavelength which can be specified. Examples of the first light include laser light, light emerging from a spectrograph, and light which has a strong spectrum in a certain wavelength region and thus can be regarded as having a single wavelength.
A seventh aspect of the invention is based on the first aspect described above and characterized in that the spectrum of the first light is used as information about the first light. Where the first light consists of two wavelengths of light having different spectra but the same amount of light, different total amounts (.SIGMA.h.nu.) of photon energies are produced. As a result, these two kinds of light write different kinds of information into the material.
A special situation in which they have different spectra but the same total amount of light (.SIGMA.h.nu.) may also be conceived. In this case, if their amounts of light are the same, it follows that the two kinds of light cannot be distinguished.
When two kinds of light having different wavelengths are used, it can be said that they have different spectra.
An eighth aspect of the invention is based on the first aspect described above and characterized in that the lower limit of wavelengths of the first light and the upper limit of wavelengths of the second light are determined by the photosensitivity of the material.
As already described in the Background of the Invention, the first aspect of the invention and associated aspects make use of the fact that the material has photosensitivity. Accordingly, where light having a wavelength to which the material does not respond cannot be used as the first or second light, expected results cannot be derived. Consequently, it is advantageous to select the first and second kinds of light from the wavelength region to which the material exhibits a practical photosensitivity.
For example, as shown in FIG. 1, a thin polycrystalline diamond film fabricated by microwave CVD in the presence of a magnetic field has a photosensitivity to a wavelength region from about 1.3 eV corresponding to a wavelength of about 950 nm to about 6.2 eV corresponding to a wavelength of about 200 nm. In this case, the first and second kinds of light can be selected from wavelengths contained in this region. It is to be noted that the value of photosensitivity determining the lower limit of wavelengths of the first light is not coincident with the value of photosensitivity determining the upper limit of wavelengths of the second light.
A ninth aspect of the present invention is based on the first aspect described above and characterized in that the material is diamond and that the lower limit of wavelengths of the first light and the upper limit of wavelengths of the second light are gamma (.GAMMA.) rays or X-rays and visible light or infrared light, respectively.
It is known that natural diamond has a photosensitivity even to radiation containing gamma rays and X-rays. Therefore, we think that diamond fabricated by CVD or a high-pressure method responds to gamma (.GAMMA.) rays and X-rays similarly to natural diamond.
Our experiment has demonstrated that a thin polycrystalline diamond film having a thickness of 15 .mu.m and fabricated by a CVD process has a photosensitivity to infrared radiation having a wavelength of 1200 nm. Therefore, we conclude that the upper limit of wavelengths of the second light is in the infrared region.
A tenth aspect of the present invention is based on the first aspect described above and characterized in that the lower limit of wavelengths of the first light is in the UV range and that the upper limit of wavelengths of the second light is in the infrared region.
Except where a material having a wide bandgap such as diamond is used, the lower limit of wavelengths of the first light can be selected from the UV range or from the X-ray range or even from gamma rays. Of course, the lower limit of usable wavelengths may be limited by the material, the method of fabrication, and the presence or absence of an additive. In this case, it is necessary to determine the lower limit of the wavelengths empirically.
Preferably, the upper limit of wavelengths of the second light is in the infrared region. Of course, the accurate value of the upper limit of wavelengths of the second light differs according to the used material. Therefore, when the upper limit of wavelength is set in practice, it is necessary to judge whether a practically usable photosensitivity can be obtained. Furthermore, it is necessary to judge whether the required photocurrent is produced.
An eleventh aspect of the present invention is based on the first aspect described above and characterized in that the apparatus satisfies the following relations EQU E.sub.1 &gt;E.sub.g &gt;E.sub.2
where E.sub.1 is the photon energy of the first light, E.sub.2 is the photon energy of the second light, and E.sub.g is the energy bandgap of the material.
It is considered that light having a photon energy of E.sub.g acts as the second light simultaneously with the first light. In this case, the above relations are modified into the form EQU E.sub.1 .gtoreq.E.sub.g .gtoreq.E.sub.2
However, as shown in FIG. 1, E.sub.g has generally has no definite value, and it is impossible to determine the value. Therefore, in practical applications, E.sub.g is preferably allowed to have some range of values, and light satisfying the above relations is selected.
A twelfth aspect of the present invention is based on the first aspect described above and characterized in that the material is irradiated with the first light to write desired information into the material. That is, the desired information is carried by the first light and written into the material by means of the first light.
An example of imparting desired information to the first light is as follows. An amount 1000 .mu.Ws/cm.sup.2 of the first light is made to correspond to information A. An amount 2000 .mu.Ws/cm.sup.2 is made to correspond to information B. Non-illumination is made to correspond to information C. In this way, three kinds of information can be entered into the material. Of course, these three kinds of information can be discriminated from each other according to the value of the photocurrent induced by illumination of the second light.
A thirteenth aspect of the invention lies in an electronic apparatus for reading information about first light impinging on a material according to the electrical resistance of the material when it is irradiated with second light or according to a photocurrent induced in the material during this irradiation. The material has photoconductivity, an energy bandgap, and trap levels. The first light has photon energies greater than the energy bandgap of the material. The second light has photon energies smaller than the energy bandgap of the material. Desired information is written into the material by irradiating the material with the first light. The written information is accumulated and stored.
The thirteenth aspect of the invention is characterized in that information entered or written into the material by means of the first light is accumulatively stored in the material. Hence, the electronic apparatus acts as a memory.
For example, when the material is illuminated with the first light having a radiant intensity of 100 .mu.Ws/cm.sup.2 for 30 seconds. Information corresponding to an amount of light 3000 .mu.Ws/cm.sup.2 is stored in the material. Under this condition, the material is irradiated with the second light to induce a photocurrent in the material. The photocurrent is measured. In this way, an output corresponding to the amount of light 3000 .mu.Ws/cm.sup.2 can be obtained.
Where the radiant intensity I varies with time, I(t)dt is stored in the material as an integrated dose.
We now discuss another example. It is assumed that the material is not irradiated with the first light at all. Then, the material is irradiated with the first light with an amount of light of 1000 .mu.Ws/cm.sup.2. Under this condition, information corresponding to the amount of light 1000 .mu.Ws/cm.sup.2 is stored in the material. That is, the material is now irradiated with the second light, and the resulting photocurrent is measured. In consequence, an output corresponding to the amount of light 1000 .mu.Ws/cm.sup.2 of the first light is derived.
Then, the material is illuminated with the first light at an amount of light 1000 .mu.Ws/cm.sup.2 without emitting the second light. It follows that the material is irradiated with the first light at a total amount of light 2000 .mu.W s/cm.sup.2.
Under this condition, the material is irradiated with the second light, and the resulting photocurrent is measured. An output corresponding to the condition in which the material is irradiated with the first light at the amount of light 2000 .mu.Ws/cm.sup.2 can be produced.
In particular, if the first light having an amount of light 1000 .mu.Ws/cm.sup.2 is emitted twice, their amounts of light are accumulatively stored in the material. The accumulated amount of light can be read out. In this way, a memory can be accomplished.
A fourteenth aspect of the present invention lies in an electronic apparatus for reading information about first light impinging on the above-described material from a pulsed photocurrent according to the electrical resistance of the material when it is irradiated with second light. The material has photoconductivity, an energy bandgap, and trap levels. The first light has photon energies greater than the energy bandgap of the material. The second light has photon energies smaller than the energy bandgap of the material. Desired information is written into the material by irradiating it with the first light. The written information is accumulated and stored. Desired information can be fetched from the information accumulatively stored. When a pulsed light is used as the second light, such a pulsed photocurrent is obtained as an output.
In the fourteenth aspect described above, fetching of desired information means that information corresponding to an amount of light 1000 .mu.Ws/cm.sup.2 is erased from information corresponding to an amount of light 2000 .mu.Ws/cm.sup.2 entered by illumination of the first light. In this case, therefore, information remaining in the material corresponds to the amount of light 1000 .mu.Ws/cm.sup.2 of the first light.
In order to fetch information from the material, it is irradiated with the second light having a given amount of light. As previously described in the Background of the Invention in connection with FIGS. 3 and 7, information written into the material with the first light is reduced not a little by performing reading operations using illumination of the second light. Utilizing this phenomenon, information stored in the material can be erased. It is possible to erase the information partially rather than totally. That is, only desired pieces of information can be erased by irradiating the material with light having such an amount of light or wavelengths which can erase the desired pieces of information. The light must satisfy the conditions for the second light.
In the above example, information corresponding to the first light having the amount of light 1000 .mu.Ws/cm.sup.2 can be fetched from information corresponding to the amount of light 2000 .mu.Ws/cm.sup.2 already stored in the material by erasing information corresponding to the amount of light 1000 .mu.Ws/cm.sup.2. In this structure, desired information can be written and read. Specifically, information corresponding to the amount of light 1000 .mu.Ws/cm.sup.2 can be written into the material by irradiating the material with the first light having an amount of light 1000 .mu.Ws/cm.sup.2. Furthermore, the information corresponding to the amount of light 1000 .mu.Ws/cm.sup.2 can be taken out from the material by illuminating the material with the second light having such an amount of light which erases the amount of light 1000 .mu.Ws/cm.sup.2.
The following arithmetic operations can be performed by making use of the principle described above. The amount of light 1000 .mu.Ws/cm.sup.2 of the first light is made to correspond to one unit of information. First, the material is irradiated with the first light to write 5 units of information (corresponding to an amount of information 5000 .mu.Ws/cm.sup.2 of the first light) into the material. Then, the material is illuminated with the second light to fetch 2 units of information from the 5 units of information. As a result, 3 units of information remain in the material.
In the above description, the treated information is made to correspond to the amount of the first light. The information may also be made to correspond to various wavelengths of light. In this case, it is assumed that light having a wavelength of x.sub.1 nm indicates information A and that light having a wavelength of x.sub.2 nm indicates information B.
Also, combinations of various amounts and various wavelengths of the first light can be used to indicate various kinds of information. In this case, the first light having a wavelength of x.sub.1 nm and an amount of light y.sub.1 .mu.Ws/cm.sup.2 is made to correspond to information A. The first light having a wavelength x.sub.2 nm and an amount of light y.sub.2 .mu.Ws/cm.sup.2 is made to correspond to information B.
A fifteenth aspect of the present invention is based on the first aspect described above and characterized in that the material is irradiated with the first light of an amount which compensates for a decrease in the information stored in the information, the decrease taking place spontaneously or being caused when the material is irradiated with the second light.
In the fifteenth aspect of the invention, the material is irradiated with the first light to write desired information into the material. As already described in conjunction with FIGS. 3 and 7, the amount of information stored in the material decreases more or less by performing reading operations. Accordingly, if the amount of information lost by one reading operation is known, then the lost information can be recovered by illumination of the first light. That is, if information about the lost information is written, then the apparent amount of information is retained constant.
As mentioned above, desired information is written by illumination of the first light and desired information can be taken out or erased by illumination of the second light. This makes use of the fact that illumination of the second light causes a part of the information stored in the material to be taken out. Conversely, information corresponding to the information taken out can be written into the material by illumination of the first light.
An example of operation is described now. First, five pieces of information A.sub.1 =100 .mu.Ws/cm.sup.2, A.sub.2 =200 .mu.Ws/cm.sup.2, A.sub.3 =300 .mu.Ws/cm.sup.2, A.sub.4 =400 .mu.Ws/cm.sup.2, and A.sub.5 =500 .mu.Ws/cm.sup.2 are established, corresponding to amounts of the first light. It is assumed that information A.sub.1 is stored in the material. The material is irradiated with the second light, and the resulting photocurrent is measured. In this way, the information A.sub.1 is read out. If the subsequent reading operation is performed, the amount of the photocurrent will be reduced more or less.
Of course, if weak pulsed light having a long wavelength is used as the second light, the difference between the photocurrents respectively induced by two reading operations is so small that it can be neglected. That is, the decrease in the photocurrent encountered in the second reading operation can be almost neglected.
We assume that information corresponding to the amount of light 20 .mu.Ws/cm.sup.2 of the first light is lost due to the operation for reading out the information A.sub.1. It is also possible to consider that the information is erased or taken out. Specifically, if the information A.sub.1 is read out and subsequently the same information A.sub.1 is again read out, then the output, or the photocurrent induced by the second light, produced during the second reading operation corresponds to an amount of light 80 .mu.Ws/cm.sup.2 of the first light.
Accordingly, after the information A.sub.1 is read out, the material is irradiated with the first light having an amount of light of 20 .mu.Ws/cm.sup.2, so that the information A.sub.1 is retained. Other information can be retained in the same way. Where each different piece of information is treated in this way, it is desired to previously measure the amount of decrease of information when each different piece of information is read out.
Our fundamental experiment has proved that information once written decreases if it is allowed to stand for a long time, or tens of hours. Therefore, where a writing operation and a reading operation are performed at a long interval, it is necessary to compensate for the loss. Also in this case, it is desired to previously measure the amount of loss produced after a lapse of a given time and to irradiate the material with the first light having the amount of light corresponding to the amount of loss so as to retain the information. The intervals at which the material is illuminated with the first light to retain the information stored in the material are set to microseconds to tens of hours, depending on the manner in which information is written and read and on the kind of the material.
In the description made thus far, given information is retained by illuminating the material with the first light having the corresponding amount of light. The means for writing desired information into the material is not limited to the amount of the first light. For example, desired information may be written by changing the wavelength. The operation described above may be carried out by using light of an appropriate wavelength having an appropriate amount of light as the first light.
Let us assume that the first light acting as writing light has a wavelength of x.sub.1 nm and an amount of light of y.sub.1 .mu.Ws/cm.sup.2. In the above description, lost information is compensated for, using light having a wavelength of x.sub.1 nm. However, in principle, light having a wavelength (e.g., x.sub.2 nm) different from the wavelength x.sub.1 nm can act in the same way if the light satisfies the conditions for the first light. In this case, the amount of light of the wavelength x.sub.2 nm is so set that carriers approximate in number to the carriers produced in the case of illumination of the wavelength x.sub.1 nm may be trapped in the trap levels in the material.
A sixteenth aspect of the invention is based on the first aspect described above and characterized in that the material has a junction between dissimilar conductivity types or a Schottky junction.
Photodiodes and phototransistors are well known as devices for detecting light. These devices efficiently take out carriers produced in response to light illumination, by making use of the action of an internal electric field produced by a junction between dissimilar semiconductors such as a PN junction.
A straightforward method of obtaining a photocurrent is to apply a bias voltage to the material as described above. A method of efficiently directing carriers optically generated to electrodes can utilize a junction between dissimilar conductivity types. Examples of the junction between dissimilar conductivity types include PN junction, IN junction, IP junction, and PIN junction. Besides the junctions between dissimilar conductivity types described above, a Schottky junction 201 (as shown in FIG. 2(d) can be used.
A seventeenth aspect of the invention lies in an electronic apparatus having a material which has photoconductivity, an energy bandgap, and trap levels. The material is provided with a plurality of device regions into which desired information can be written. The electronic apparatus is further equipped with a means for irradiating the device regions with first light, a means for irradiating the device regions with second light, and a means for measuring a pulsed current induced in the device regions. The first light has photon energies greater than the energy bandgap of the material. The second light has photon energies smaller than the energy bandgap of the material.
In the aforementioned seventeenth aspect, each one device region can be regarded as an electron apparatus defined by the first aspect. Therefore, the structures of the other aspects described thus far can be applied to these device regions. As described above, the electronic apparatus can act as a sensor, a memory, or a calculator for treating information corresponding to amounts of the first light, information corresponding to various wavelengths of the first light, information corresponding to combinations of amounts of the first light and various wavelengths, or information corresponding to various spectra of the first light. In the seventeenth aspect, the electronic apparatus is equipped with a plurality of devices having such functions. Hence, the apparatus can treat complex information. Therefore, the components are similar to the components described already unless otherwise stated.
In the seventeenth aspect described above, "the means for measuring the pulsed current induced in the device regions" comprises a pair of electrodes mounted on the device regions, a bias voltage source for applying a bias voltage between the electrodes, a voltmeter for converting an induced electrical current into a voltage and measuring the voltage, a determining means for determining the value of the photocurrent from the voltage measured by the voltmeter, a means for integrating the value determined by the determining means, and other requisite arithmetic units and memories.
An eighteenth aspect of the invention lies in a method comprising the steps of: irradiating the device regions with first light to write desired information into the device regions; and irradiating the device regions with second light to take out or read desired pieces of information from the device regions.
To take out desired pieces of information is to erase desired pieces of information by irradiating the material with the second light or light satisfying the conditions for the second light. The irradiation of the second light excites carriers trapped in the trap levels in the device regions, thereby reducing the amount of information retained.
To read desired pieces of information is to read information, for example corresponding to the amount of the first light, by irradiating the material with the second light and measuring the induced photocurrent.
The above-described structure permits fabrication of an electronic apparatus capable of writing desired pieces of information into plural device regions and reading or taking out desired pieces of information from the device regions. This electronic device is characterized in that information can be accumulatively stored in the device regions. Also, the information can be taken out, or erased, and written at will. In addition, accumulatively stored information can be read out.
An example of the structure in which plural device regions are formed on the material is shown in FIG. 8, where a plurality of device regions A.sub.mn are arranged in rows and columns on the surface of a thin-film material such as a thin-film polycrystalline diamond.
We now discuss a situation in which one device region treats information corresponding to the amount of the first light. The amount of light 1000 .mu.Ws/cm.sup.2 of the first light is taken as a unit of information. Information from 0 unit to three units is treated. In particular, 0 unit of information corresponds to the state in which the device region is not irradiated with the first light. One unit of information corresponds to the amount of light 1000 .mu.Ws/cm.sup.2 of the first light. Two units of information correspond to the amount of light 2000 .mu.Ws/cm.sup.2 of the first light. Three units of information correspond to the amount of light 3000 .mu.Ws/cm.sup.2 of the first light. In this case, if one unit of information is written twice consecutively, then two units of information are written into the device region. If the device region in which the two units of information are stored is irradiated with the second light to take out one unit of information, then the information remaining in the device region is one unit.
In this case, four kinds of information, from 0 to 3 units, are treated in the m.times.n device regions arranged in rows and columns. Therefore, an m.times.n matrix can be calculated. Also, information represented by a two-dimensional image can be treated while taking information written into each one device region as a pixel.
The above-described structure treats information about a matrix array. If each one device region is several micrometers square, each one device region can be treated as a pixel. In this way, image information can be created. We now discuss a situation in which three units of information are treated on the device regions although these units of information cannot be recognized visually. One unit of information is made to correspond to black. Two units of information are made to correspond to a halftone between black and white. Three units of information are made to correspond to white. These units of information are combined on a two-dimensional plane, whereby information about a black-and-white image is created.
Also, plural pieces of image information can be subjected to an additive or subtractive operation. This corresponds to summation of plural pieces of image information and difference between them. Using these operations, plural pieces of information about image can be arithmetically operated.
Where information about a matrix or an image is arithmetically treated, an additive operation is performed by irradiating the material with the first light. A subtractive operation is performed by irradiating the material with the second light.
A summation of information is to accumulate information in one device region. This is realized by irradiating the material with the first light. A subtraction is to take out information from the device region. This is achieved by irradiating the material with the second light. Of course, the amount of the first light or the amount of the second light necessary to add or subtract desired information is required to be predetermined. For example, the amount of the second light necessary to take out information corresponding to the amount 1000 .mu.Ws/cm.sup.2 of the first light is required to be set in advance.
In the above-described structure, light for reading information may be different from light for taking out information. For example, in order to minimize the decrease in the amount of information caused by a reading operation, the reading light has a longer wavelength, while the light used to take out information has a shorter wavelength, for obtaining a desired photon energy.
Let us assume that information stored in one device region is indicated by z-axis and that a plane formed by plural device regions is indicated by x- and y-axes. Thus, information expressed by a three-dimensional image can be treated.
Weighted pieces of information can be accumulatively written into each one device region. Device regions formed on a two-dimensional plane create two-dimensional information. Information stored in each device region is used as one-dimensional information. In this way, information corresponding to a three-dimensional image can be created. Each piece of information about the three-dimensional image can be added to or subtracted from other information.
It is advantageous to arrange means for emitting the first light in rows and columns in conformity with the matrix arrangement of the device regions.
The speeds at which information is written and read can be effectively increased by providing light-emitting means corresponding to the device regions, respectively. That is, where the device regions are arranged in rows and columns, arithmetic operations can be performed at higher speeds by arranging first light-emitting means also in rows and columns.
The speeds of arithmetic operations can be further increased by arranging second light-emitting means in rows and columns.
The first or second light-emitting means arranged in rows and columns can be semiconductor lasers corresponding to the device regions, respectively, or optical fibers or optical waveguides for guiding light from one or more light sources to the device regions.
A nineteenth aspect of the present invention lies in an electronic apparatus for irradiating a material with first light and then irradiating the material with second light to induce a pulsed photocurrent in the material. The apparatus is characterized in that it utilizes the fact that the amount of the first light is in proportion to the pulsed photocurrent. The material has photoconductivity, an energy bandgap, and trap levels. The first light has photon energies greater than the energy bandgap of the material. The second light has photon energies smaller than the energy bandgap of the material.
The nineteenth aspect described above makes use of the fact that the amount of light impinging on the material is in proportion to the peak value of the photocurrent induced by the second light impinging on the material or to a value obtained by integrating the photocurrent for a given time, as shown in FIG. 6. This aspect can be applied to photosensors, optical memories, and optical arithmetic devices.
The nineteenth aspect is also characterized in that information about the first light having a wavelength corresponding to an energy higher than the energy bandgap of the material is obtained from a change in a physical change in the material caused by illumination of the second light having a wavelength corresponding to an energy smaller than the energy bandgap of the material.
The change in the physical property of the material are attributed to carriers excited by the energy of the second light in the material. The change in the physical property of the material brings about a change in the electrical resistance of the material, or a photovoltaic effect. In order to electrically read the change in the physical property, the material is irradiated with the second light while a bias voltage is applied between a pair of electrodes. A pulsed photocurrent induced across the electrodes is measured.
First, the material is irradiated with the first light having a wavelength corresponding to an energy higher than the energy bandgap of the material. At this time, information about the first light is stored in the material.
During or after this irradiation, the material is irradiated with the second light having a wavelength corresponding to an energy lower than the bandgap of the material. A pulsed photocurrent induced at this time is measured. In this manner, information about the first light impinging on the material before the measurement can be obtained. Especially, with respect to the amount of the first light, the amount can be read accurately.
Also, desired information can be taken out, or erased, by irradiating the material with the second light. By utilizing this, information can be written by irradiating the material with the first light. Information can be taken out by irradiating the material with the second light. In this manner, addition and subtraction of information can be done. Information stored in the material can be read by measuring the pulsed photocurrent induced by the illumination of the second light.